Many structures, including buildings such as homes, offices, retail space, and manufacturing space, are built with at least a portion of the building in direct contact with soils. Soils provide a base or platform on which the building can rest that can serve to support the building. Soils can exhibit fluid characteristics, and as a consequence, a solid base such as a foundation, is generally provided as part of building construction. While a foundation may provide a more stable substructure than bare soil, the fluid properties of soils can compromise a foundation, or cause the foundation to fail. Many different types of soils are encountered in different geographic locations and in different building situations, which can require adaptations so that the building foundation interacts with the soil in such a way as to provide adequate support and reduces, minimizes, or maintains relative movement of the building and the soil within acceptable tolerances.
When relative movement between a building and the soil upon which the building is built or rests is exposed to, or undergoes, excessive relative movement, stress (force per area) develops on the building and can result in strain (deformation per unit length), movement, shifting, and breakage of the building, including the foundation. Movement of soils can occur quickly such as with earthquakes and liquefaction, or more slowly, as with heaving and settling. Repairs relating to structural foundation problems amount to roughly $55 billion a year in the United States. In fact, in some areas, such as the greater Phoenix Metro Area of the State of Arizona, roughly half of remodels that involve additions or expanding a footprint of a building experience foundation problems, which can lead to costly repairs.
FIG. 1A shows a cross-sectional view of a portion of a structure or house 10 that is built using slab on grade construction. Structure 10 can comprises footings 12 and stem walls 14 that together form foundation 16. Footing can be made or concrete reinforced with steel, such as rebar. Stem walls 14 can similarly be reinforced concrete, or alternatively can be masonry or block. Together, foundation 16 can support a superstructure or a balance of structure 10 including walls 18 and a roof 20. Both walls 18 and roof 20 can be constructed of lumber. Alternatively, walls 16 can be constructed or masonry, block, or any other suitable material.
Foundation 16 can be disposed in, and supported by, native soil 124. Soil 24 can also provide support for floor slab 26. Slab on grade construction include a concrete floor slab 26 that can be poured, formed, or built within a perimeter formed by the stem wall 14. Floor slab 26 can be in contact, and often direct contact, with leveled or graded soil. The graded soil can be formed as a prepared pad of soil that has been compacted for stability and built to a particular elevation or grade to account for drainage away from the building and other issues. Advantageously, an intermediate layer of engineered soil or an aggregate base course (ABC) 28 comprising rock, sand, and dirt can be deposited, graded, wet, and compacted over native soil 24 before placing and finishing concrete floor slab 26. ABC layer 28 can generally comprises a thickness in a range of 7.6-15.2 centimeters (cm) or about 10.2 cm (or 3-6 inches (in.), or about 4 in.). The placement and use of ABC layer 28 between native soil 24 and floor slab 26 reduces soil movement and attendant cracking of floor slab 26. Floor slab 26 can be formed of a layer of concrete that can generally comprises a thickness in a range of 7.6-15.2 cm or about 10.2 cm (or 3-6 in., or about 4 in.).
FIG. 1B shows a cross-sectional view of a portion of a structure 10, similar to the view shown in FIG. 1A. FIG. 1B provides an illustration of structural damage that can result from upward movement or heaving of native soil 24 when the native soil is or comprises an expansive soil 30, such as clay. When expansive soil 30 becomes wet or increases in moisture content, the expansive soil swells and increases in size so that a top surface of the soil moves upward. When soil is constrained on its upper surface, such as by structure 10, the soil can lift, shift, and move footings, stem walls, floor slabs, as well as walls and roofs attached to the footings, stem, and slab. Excessive movement, especially differential movement, of various portions of structure 10 can cause cracking and failure of the various portions. FIG. 1B shows a broken floor slab 32 comprising uneven surface 34 and cracks 36 that were caused by the uplift of heaving soil 30. While the heaving of expansive soil has been shown with respect to uplift caused by the moisture content of a dry expansive soil increasing, the opposite can also occur. In situations where the moisture content of a wet expansive soil decreases, soil shrinkage and settling can occur with similar results of differential movement and structural damage.